So yesterday I bought the aquatop 100 w heater for my 20 gallon long aquarium. I followed the instruction manual and also placed the heater near air stone so it has circulation. So what is happening is the heater, at least from what I understand so far, is that I'll turn the heater on let's say at 76 degrees, the heater will run past 76 to the 80s, I'll slowly turn the nob down and wait and the heater won't shut off. So now with the water at like 82 I unplugged it for about 30 minutes to let it re-accumulate to the water temp, I plugged it in set it to about the desired temp, it didn't kick on so I fiugured it would when the temp lowered. Anyway I came home later and the water was at 72 and the heater wasn't on. Any suggestions on what to do to try to set the heater or should I try taking it back. Thank You